10k:50k scales 15volt down to 1/6 = 2.5volt.
That's using 50% of the A/D range with default Aref (5volt). So downgraded 10-bit to 9-bit.
The trick is to use the right resistor ratio, so you're using max range of the A/D.
Did you watch the video. Did you see the display jump op to 0.1volt.
Reason: default Aref instead of 1.1volt Aref, wrong resistor ratios, and zener diode (very bad).
If you do it right, then you can reduce the jitter to ±0.01volt.
Leo..